New release checklist

1. Bump up the version number in nflows/version.py and download_url field in setup.py.
2. Create a new GitHub release at https://github.com/bayesiains/nflows/releases, for a new tag named vX.XX.
3. Run python setup.py sdist.
4. Run twine upload dist/*, using PyPI credentials when asked.
5. Test that worked: pip install nflows --upgrade.
6. Check Zenodo release has been created: https://zenodo.org/deposit. Edit if necessary.
7. Update Zenodo links in the README.md.

More details: https://medium.com/@joel.barmettler/how-to-upload-your-python-package-to-pypi-65edc5fe9c56